no, I think you're a bit wrong.
you're not telling them that they are your rival. no no no. (no same person will do that cuz he'll loose their element of surprise and lower the chances of winning)
you're telling everyone around you in the galaxy that THIS GENOCIDAL EMPIRE is now officially your RIVAL. and they'll have to deal with you from now on. (this will lead to a respect, open some relationship and explain how you actually gain influence)
Edit:
I imagine it's something like this:
if there is a Big Bad Dragon that steals your cattle and princesses to eat... you will not go to this cave and declare that soon™ you'll kill him (you just do it if you can)
but instead you can tell everyone that you'll soon go to kill that dragon ..and enjoy the support and free food from the grateful citizens around(finally there is a fool that'll do it for us... or die trying)
and it even doesn't require you to actually go to the cave and deal with a dragon.... just tell them that you'll do it "Soon™"